The present invention relates generally to television signal transmission and receiving systems and, more particularly, to a system wherein the horizontal synchronization components of a conventional NTSC television signal are suppressed providing a scrambled transmission format particularly useful in subscription or pay-television applications.
In subscription television systems programming signals are typically transmitted, either "over-the-air" or through a suitable cable network, in a scrambled form rendering the broadcast video information largely unviewable when received by a conventional television receiver. In order to unscramble the video display, each system subscriber is provided with a decoder operable for unscrambling the broadcast signals and for coupling the unscrambled signals to a conventional television receiver for viewing. Security is, of course, a prime consideration in the design of such systems. That is, the scrambling technique employed should be of a nature making the unauthorized decoding or unscrambling of the broadcasts difficult in order to deter the would-be pirate. At the same time, the scrambling technique must accommodate reliable decoding by authorized system subscribers.
One technique commonly used for scrambling the video display produced in response to a broadcast television signal is that of horizontal sync suppression. Suppression of the horizontal synchronization components of a broadcast television signal below most video levels will cause the deflection circuits of a normal television receiver to behave eratically such that a scrambled video image is produced on the display screen of the receiver. In addition, the ability of the television receiver to use the color reference burst associated with the horizontal synchronization signals is severely degraded thereby causing inaccurate color reproduction.
Exemplary prior art sync suppression systems are disclosed in U.S. Pat. Nos. 3,184,537 to Court et al; 3,478,166 to Reiter et al; 3,530,232 to Reiter et al and 4,222,068 to Thompson. In these prior art systems, the horizontal synchronization components of a broadcast television signal are typically suppressed or reduced to gray level and an additional keying or control signal is normally transmitted together with the television signal for controlling re-construction or regeneration of the proper horizontal sync levels at the receiver. For example, in U.S. Pat. No. 3,184,537 an audio sub-carrier is amplitude modulated with a suitable sync-insertion control signal. In other cases, horizontal sync re-construction at the receiver is effected by transmitting normal sync signals during the vertical interval of the television signal for enabling a timing circuit to lock to the horizontal components thereof. The timing circuit may then be used to accurately define the horizontal blanking intervals of the upcoming field to facilitate restoration of the horizontal synchronization signals in the composite baseband video signal.
It is a basic object of the present invention to provide an improved horizonal sync suppression system of the type especially useful in a subscription television system.
It is a more specific object of the invention to provide a horizontal sync suppression system for scrambling a broadcast television signal in a manner so as to deter the unauthorized decoding of the scrambled broadcast.
It is a further object of the invention to provide a horizontal sync suppression system for scrambling a broadcast television signal which does not require the transmission of additional control signals nor the use of complex timing circuits to effect restoration of proper synchronization signals at a television receiver.
It is yet another object of the invention to provide a horizontal sync suppression system for scrambling a broadcast television signal in which the decoding or unscrambling process is not necessarily accomplished at baseband.